Category Archives: Radio gear

Video blog episode 011

I finally finished off the 6-meter Yagi project and got it on the air. I’ve actually been using it for a while, but needed the ARRL VHF contest to get enough activity on the band to shoot video in a reasonable amount of time showing me making QSOs. I was running 50 watts into the 4-element Yagi and worked sporadic-E clouds to 30 different Maidenhead grids, including two in New Hampshire, which were my first double-hop sporadic-E QSOs. Enjoy!

Video blog episode 005

Now that I’ve been working on ham radio antennas, I have a couple of projects for the video blog. The first one is a 6-meter 4-element Yagi. It is designed to be semi-portable, and for me this is particularly important because it’s not practical for me to leave up an antenna for any significant length of time. Perhaps impudently, I am posting this introduction and design video before I actually have the antenna on the air! However, I have tested the antenna itself (sans 1:1 balun) with a VNA and it seems to behave more or less as expected.
I spend a lot of time talking about various design parameters and the actual modeling procedure, so this may seem a little dry if you’ve done this before. But, I try to keep my speech pace relatively slow on these videos, so there’s always the 1.25x and 1.5x speed options on YouTube.  Part II will detail the construction process, and Part III will show testing and hopefully actual on-air performance.

Video blog episode 002

Episode 2 of my video blog has been posted to YouTube. This is the second video in the Superloop antenna series, and shows the construction of a decade resistance box to use as the terminating resistance of the antenna. The box allows you to repeatably set the resistance to the value you want, as opposed to trying to make fiddly adjustments of a potentiometer. This part of the project went well, other than it having a 2 ohm offset due to resistance in the thumbwheel switches, wires, and banana plug connectors. Enjoy!

Video blog debut!

Finally! My first Arizona Signal Watcher video blog episode is now available on YouTube. This is the first of a several-part series on the “Superloop” receiving antenna. This design was developed by Bruce Conti and this is my take on the design and construction. The antenna is very useful and convenient when needing a signal null in one direction and good reception in other directions, and when portability is important. The first video is entitled “Superloop Antenna Part I: Introduction” and gives an overview of the antenna design and some theory behind it. Subsequent videos will detail the construction of the antenna and the electrical/electronic components needed to optimize the antenna performance. Comments here or on the YouTube page are very much welcomed. Enjoy!

 

How-to: Aluminum case for new RTL-SDR blog brand TCXO dongle

The RTL-SDR Blog posts articles and links related to software defined radio. They also sell RTL-SDR dongles and accessories. Last month they introduced a dongle with a temperature compensated oscillator (TCXO). This virtually eliminates the well-known offset and frequency drift of “standard” dongles as they heat up or the ambient temperature changes. NooElec also makes a model like this, but if you don’t like the MCX connector on the latter, the RTL-SDR Blog version uses an SMA connector (and is a bit cheaper at the moment). Both brands are good products in my experience.

The RTL-SDR Blog does not sell an aluminum case for this dongle (yet), but once you get past your first RTL-SDR experimentations, this is a must have. A metal case shields the dongle from RF interference and provides a better ground plane. It is a fairly straightforward mod to get the RTL-SDR blog dongle to fit in the RTL-SDR case sold by NooElec.

Disclaimer: you can potentially ruin your RTL-SDR dongle, so attempt this at your own risk.

Tools needed: sandpaper, small metal file, small phillips screwdriver, power drill, hammer and hole punch (optional)

Here is a picture of a modified case and an unmodified case, plus the file I used, and the dongle removed from the plastic case it comes with:
IMG_1574_case_blowup

I did not take a picture of an unmodified dongle, but you want to sand down the long edges of the PCB just enough so that it fits on the rails in the case. Normal fine grit paper works fine. Also, you can sand down the PCB that sticks out from where the USB plug is mounted. Do not sand too much or you can hit conductive traces and/or solder points.

Using the file, etch smaller slots on the short sides of the existing USB slot in the case. You might also need to file down one of the long sides of the main slot to allow the plug to vertically fit. You want to do this fairly precisely to avoid creating a large hole for RF interference to leak in, so trial and error is the best method.

On the other end cap, you will drill a 1/4″ hole for the SMA plug. Note that a commercial case might have a square hole to accommodate the lip of the plug, but that it not necessary. It just means that the USB plug will stick out a couple millimeters more than usual on the other end. Use the slot end of the case as a template to determine where to drill the hole. A hole punch can be very helpful here to mark the hole location to keep it in the same vertical plane as the rest of the dongle and USB plug.

Some people claim that the paint on the case can cause electrical problems, i.e., the case will not be a unified ground plane. So, before you put the case together, use the file or sandpaper to strategically remove paint where the case pieces fit together. This does not have to be precise.

You can fill the MCX hole in the case with a short screw and nut to prevent possible RF interference from entering here.

Here are a couple shots of the end product (a little blurry, but you get the point):
IMG_1571_USB_end

IMG_1572_SMA_end

I’ve had this running for about 20 hours total since I made the mod and it works fine.

Gear: A filtering option for strong stations on upper AM broadcast band

I will be writing more about broadband antennas for AM BCB in the future, but one of the issues I have is the dual problem of using an inexpensive low dynamic range RTL-SDR dongle as a receiver and having two local stations right next to each other on the dial.  Granted, this is a much easier problem to deal with than in a big city where there might be many strong stations spread across the dial.  At least if one’s worst problem stations are at either the low end or high end of the dial, filtering becomes a practical option.

I live within 2 miles of the transmitters for 1 kilowatt stations on 1450 and 1490 kHz.  While I’m lucky in that the station on 1490 only broadcasts from 5am to midnight even though it is licensed for 24-hour operation, with the stations so close to each other, the frequencies can interact and create intermodulation products at 40 kHz intervals below 1450 and above 1490. (In fact, 40 kHz is the closest allowed separation for stations serving the same community; why our only two AM stations are this close on the dial is beyond me.)

Anyway, when using an untuned loop antenna for broadband coverage, the signal tends to be quite weak, requiring significant amplification to get decent signals at all broadcast frequencies and to pick up trans-Pacific or trans-Atlantic signals if so desired.  But, strong local signals can overload with amplification and cause problems at other frequencies.  I find that I can’t otherwise amplify a broadband loop such as a Conti Superloop due to the strong 1450 and 1490 signals. A more expensive SDR with a higher bit rate A/D converter would probably help with the dynamic range, but this seemed like a good opportunity to experiment with filtering.

It’s possible to build one’s own filters out of discrete components, but the frequency precision required for something like this makes the project quite challenging. Thus, I went with off-the-shelf filters from Mini-Circuits. They seem to have the best options at lower frequencies like this. I narrowed it down to two filters, the LPF-B0R7+ and the LPF-B0R8+. These are both surface mount filters as opposed to “plug-and-play” filters. Unfortunately, they have to be purchased in bulk, at least 10 units. Thus, the cost is about the same ($80-90) as a cased filter with connectors. (Note that the plus sign at the end of the part number just means that it satisfies the international RoHS guidelines for hazardous substances.)

The following plot uses the data available on the Mini-Circuits site, showing from top to bottom the response of a single LPF-B0R8 filter, two LPF-B0R8 filters in series, and a single LPF-B0R7:
LPF-B0R7_vs_LPR-B0R8
You can see that while the LPF-B0R7 kills 1450 and 1490 kHz, it also wipes out too many lower frequencies. A single LPF-B0R8 is actually pretty decent, but I wanted to make sure I really diminished 1450/1490 and had a sharper rolloff. Plus, having to buy 10 units, I had plenty to spare, so I build a double LPF-B0R8 filter. When you multiple numbers together, the logarithms add, so the loss depicted on the graph for the double filter is exactly twice the value for a single filter.

The data sheet for the filter indicates which of the solder pads on the bottom of the filter are input, output, and ground, so doubling up is just a matter of making sure that the output for one filter is connected to the input of the second filter. I put the filters on perfboard, soldering from below to get enough solder through the plated hole up to the ground pads to attach the filter to the board. One has to be a little careful not to overheat the component while soldering. I ran 20-gauge connector wire from the “hot” lead of an SMA connector to the input of the first filter, another wire from the output of the first filter to the input of the second filter, and a third wire from the output of the second filter to the “hot” lead of the output SMA connector. I then ran “ground” wires from two of the ground posts of the input SMA connector to the corresponding posts of the output connector, making sure to make a solder connection through the perfboard to the ground pads of each filter along the way. The following pictures show the result from the top and from the bottom:
IMG_1030

IMG_1031

I found a box on Amazon that was perfect for this particular 2×8 cm board, resulting in the following finished product:
IMG_1033

It turns out that the filter works pretty much exactly as expected. Here is a screenshot from the gqrx SDR software of AM BCB from 540 through 1490 kHz:LPF-B0R8_example
You can see the general signal dropoff at high frequencies along with the dropoff in noise level. The stations at 1450 and 1490 kHz are still fairly strong, but without the filter they would by far be the strongest signals in the band and at this gain level would overload the RTL-SDR dongle. The signals beyond that are basically buried in the noise, which is collateral damage in this case. However, that’s a small part of the band and there are very few stations beyond 1600 kHz in any case. I can use a tuned loop in this area to sample individual frequencies.